Pregnancy Weeks to Months
Convert pregnancy weeks to months, find the trimester and the due date
"Pregnancy Weeks to Months" Calculator
How pregnancy weeks and months are counted
What the calculator does
Enter a term in weeks — or pick a month of pregnancy — and the page says how many obstetric and calendar months that is, which trimester it falls into, how many weeks are left to the full term and when the estimated due date is.
An obstetric month is four weeks, and that is the whole confusion
An obstetric month is exactly 4 weeks, or 28 days. A calendar month averages 30.4375 days. The two counts drift apart from the first month, and by the end of a pregnancy they differ by about 0.8 of a month:
That is why a doctor says «ten months» and everyone else says «nine», and both are right. Every page of this calculator prints both numbers, because the question is almost always about the disagreement rather than about either number alone.
Where the count starts
Pregnancy is dated in completed weeks from the first day of the last menstrual period, not from conception. That is the convention the count is built on, and the calculator does nothing else with it: the two starting points are about two weeks apart, so a term counted from one of them does not match a term counted from the other.
Trimesters
- First trimester — weeks 1 to 13.
- Second trimester — weeks 14 to 27.
- Third trimester — week 28 until birth.
The boundaries are the ones used by ACOG. They fall inside obstetric months rather than on their edges, which is why a month of pregnancy can belong to two trimesters at once — the calculator says so when it happens.
How the due date is calculated
This is Naegele's rule. The calculator works backwards from the term you entered: if the term is right today, the remaining part of those 280 days lands on the estimated date. Past 40 weeks the same arithmetic puts that date in the past, and the page prints it as such, saying plainly that the full term is already behind — beyond term what happens next is a clinical question, not an arithmetic one.
What this calculator is not
It is arithmetic, not medical advice. The date it prints is an estimate produced by one formula, and a real birth can fall on either side of it. It says nothing about screenings, ultrasounds, maternity leave or any other question that depends on a doctor or on the law where you live.
See Also
- Weeks and Days Ago From Today - Calculate what day was it Weeks and Days Ago From Today
- Time Units Converter - Convert Hours/Days/Weeks/Months/Years to Seconds/Minutes/Hours
- Date Calculator - Add to or Subtract Days/Weeks/Months or Years from Today
- Days Between Two Dates - Calculate the Difference Between Two Dates. How many days, months, and years are between two dates? From date to date calculator
- Days until Date - How many days, weeks, months and years until a Date. How Many Days Until My Birthday? Countdown to date
About "Pregnancy Weeks to Months" Calculator
Pregnancy is counted in completed weeks from the first day of the last menstrual period, and a full term is 40 weeks or 280 days — that is Naegele's rule. Enter a term in weeks and the calculator says which obstetric month it falls into, which trimester it belongs to, how many weeks are left and when the estimated due date is.
An obstetric month is exactly four weeks, which is why 40 weeks make ten obstetric months but only about nine calendar ones. Both numbers are printed on every page, because the two counts disagree and the disagreement is what the question is usually about.
This is an arithmetic tool, not medical advice. The due date it prints is an estimate; only a doctor can set the real term.